The problem of trapping of the shortwave and longwave radiation in the urban canyon belongs to the one of the most important for understanding singularities of the city climate. In many models either just a first (second) reflection or a multiple exchange but only between pairs of surfaces are considered. The present work is an example of application of the analytical technique allowing to calculate an absorption of the solar radiation after infinite reflection between finite surfaces. In this example the method is applied to estimate an effective albedo of the urban canyon but it can be used to evaluate the longwave radiation exchange also. The dependence of the energy absorbed by the canyon on the street height to weight ratio was tested for different values of the surface albedo. Analysis was provided for different canyon orientation in different radiation condition: sunny day (diffuse radiation constitute only small fraction of total radiation), and cloudy day (only diffuse radiation).